Heat-shrinkable multi-layer films have heretofore been widely used in the form of vacuum shrink packages and MAP in fields of which good gas barrier property is required, such as processed meat products, fresh red meat (FRM) and cheeses. The heat-shrinkable multi-layer films are used as bag products subjected to sealing by a method such as heat, impulse or high frequency, or films subjected to packaging machines.
A representative basic structure of the conventional heat-shrinkable multi-layer films is composed of (1) a gas barrier resin such as polyvinylidene chloride (PVDC) resin or ethylene-vinyl alcohol copolymer (EVOH) resin, provided as a core layer, (2) a heat-sealing resin selected from the group consisting of polyolefin (PO) resins such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and very low density or ultra low density polyethylene (VLDPE or ULDPE), ionomer resins, ethylene-vinyl acetate (EVA) resins, etc., provided as an inner layer, and (3) a thermoplastic resin having excellent abuse resistance, slip property, package machinability, etc., provided as an outer layer. There has recently been proposed a multi-layer film provided, as a sealing resin layer, with a PO resin obtained by using metallocene catalyst (may also be referred to as single-site or Kaminsky catalyst).
However, the heat-shrinkable multi-layer films obtained by using such a known resin as describe above as a sealing resin layer have not achieved fully satisfactory performance as bag products under the circumstances. For example, the heat-shrinkable multi-layer films making use of LDPE in a sealing resin layer involve drawbacks that they have insufficient sealing strength and poor sealability through contamination because part (contamination) of the contents, which have adhered to sealing surfaces upon filling of the contents, form the cause of insufficient sealing. In the case where LLDPE is used as a sealing resin layer, the resulting heat-shrinkable multi-layer film has poor low-temperature sealability. In the case where EVA is used as a sealing resin layer, the resulting heat-shrinkable multi-layer film has insufficient sealing strength.
In the case where LLDPE or VLDPE polymerized using the conventional metallocene catalyst is used as a sealing resin layer, the resulting multi-layer film involves such problems that when the multi-layer film is extruded, motor torque in an extruder and pressure increase. Accordingly, productivity cannot be achieved. In addition, the resulting film with melt fracture (disordered irregularities) is insufficient in sealing properties such as sealing strength, fails to provide a beautifully finished seal line, impairs the appearance of a product packaged due to its poor clarity and moreover can not make the best use of the features of the polyethylene resins obtained by using the metallocene catalyst because its sealing strength becomes insufficient upon heat sterilization in the field of multi-layer films and casings having high shrinkage stress and heat shrink.